1. Field of the Invention
The present invention relates to a martensitic stainless steel that has superior corrosion resistance as well as superior cold workability and hot workability, and therefore the stainless steel is unlikely to crack during forging and has high levels of strength.
2. Description of the Related Art
A martensitic stainless steel, such as SUS440A, SUS440B, SUS440C and the like, is conventionally used for bearings, such as slide bearings and antifriction bearings, gears, bolts, dies, punches, cutting tools, etc., which require high hardness and wear resistance. This kind of stainless steel is disclosed in, for example, Japanese Unexamined Patent Application Laid-open No.2002-256397.
The above-mentioned martensitic stainless steels, such as SUS440A, SUS440B and SUS440C, include a large amount of carbon in order to achieve high hardness after heat treatment. Therefore, in comparison with an austenitic stainless steel such as SUS304 and SUS316, these martensitic stainless steels are inferior in corrosion resistance, and tend to rust when exposed for long time to an environment with large amount of moisture and/or salt. Moreover, because these martensitic stainless steels include a large amount of carbon, cracks may occur in forging or after quenching or sub-zero treatment. A coating treatment on a surface may be used as a means for preventing rust, but in some cases, adhesion of the coating material to the base material is low, or the production cost is increased.
Since eutectic carbides are generated in the above-described martensitic stainless steels, cold workability and corrosion resistance of the martensitic stainless steels are inferior to those of an austenitic stainless steel such as SUS316.
Antifriction bearings may be used in a corrosive environment. For example, antifriction bearings for submersible pumps, fishing reels, and the like, may suffer adhesion of moisture or salt. Accordingly, the material of such antifriction bearings is required to have high corrosion resistance in addition to high hardness. In this case, a corrosion-resistant stainless steel like SUS440C is commonly used. However, as described before, although the SUS440C is a stainless steel achieving high hardness after heat treatment, it still has low corrosion resistance when compared to the austenitic stainless steel such as SUS304 and SUS316. Therefore, if SUS440C is subjected to the above-described corrosive environment for long time, rust will occur, and the bearing life will be decreased.